Generate Nix expressions directly

[ttuegel]

Rather than generating a JSON file which gets processed by Nix, we should generate Nix expressions directly. One advantage of this approach is that it bypasses deficiencies in Nix's JSON parser, c.f. #2. Another advantage is that we can put package dependencies in each package's arguments so that overriding dependencies is easier, e.g.

# `foo` requires bar-2.0, but bar-1.0 is the default version
foo = super.foo.override {
  bar = self.bar-2;
}